At present extensive research has been performed on conductive paste containing a metal powder, because it is used for various purposes including fabrication of electrodes for solar cells, formation of jamper cables using electronic components, e.g., printed-wiring boards, and formation of terminal outgoing lines of printed resistors.
For example, in the fabrication of surface electrodes of solar cells, conventionally, an antireflection layer formed on a semiconductor layer is patterned using photoresist, then a surface electrode is fabricated (patent reference 1). However, this method is very complicated because it requires two steps: patterning of an antireflection layer, and fabrication of an electrode.
To eliminate the patterning process of antireflection layers, a method was proposed, wherein an antireflection layer is formed only on an light absorption side by masking the semiconductor layer during the formation of the antireflection layer which is the process prior to the patterning process. However, this method of direct patterning of antireflection layers has technological difficulties, and is not yet sufficient for practical application. Meanwhile, as a study on the fabrication of surface electrodes of solar cells, a so-called fire-through method has been proposed (patent reference 2), in which after the easy formation of an antireflection layer using a method such as a thermal oxidation method, conductive paste comprising a metal powder and a glass material is printed on the antireflection layer, then the metal powder is in contact with the n- or p-layer silicon of the silicon substrate to form an electrode while the glass material contained in the conductive paste melts the antireflection layer, so that the conduction between the metal electrode and the n- or p-layer silicon can be ensured. However, because this fire-through method requires a baking process generally at a high temperature of 850° C., the glass component and the electrode sometimes penetrate the n-layer, resulting in the deterioration of electrical properties of solar cells; therefore, accurate control of the fabrication conditions is necessary. In addition, when there is no uniform conduction between the electrode and the n-layer silicon, initial electrical properties of solar cells deteriorate, leading to a problem.
Thus, the development of functional paste having a superior etching function and good electrical properties has been strongly desired in the art.
Patent reference 1: Patent Application No. 2000-49368
Patent reference 2: Patent Application No. 2002-176186